Most people think that when you change out a master all one has to do is bleed the front and rear brakes and it will push out all the air. If you were to look at a master cutaway you would notice that the ports which feed the fluid are not strait forward of the piston but to the side of the piston. This leaves a void for air to collect at the extreme forward area of the cylinder. You can bleed the brakes all day long and never get to that air pocket. This is the reason you have to bleed the master cylinder first. The switches used regardless if used for a safety start or brake light operate with pressure of the fluid. As you press down the pedal the fluid pressure closes the switch allowing current to flow from the switch for either starting or brake light operation. If you have an air bubble at the switch, there is not enough pressure to close the switch since the air is never going to compress as much as the fluid, this is referred as air lock. Further, after bleeding the master cylinder you will always start bleeding from the furthest from master to nearest ie. rear than right front. left front.
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